Network Working Group Sean Turner, IECA
Internet Draft Dan Brown, Certicom
Intended Status: Informational February 3, 2010
Expires: August 3, 2010
Elliptic Curve Private Key Structuredraft-turner-ecprivatekey-04.txt
Abstract
This document specifies the syntax and semantics for conveying
Elliptic Curve (EC) private key information. This syntax and
semantics defined herein are based on a similar syntax and semantics
defined in Standards for Efficient Cryptography Group (SECG).
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
This Internet-Draft will expire on August 3, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
Turner & Brown Expires August 3, 2010 [Page 1]

Internet-Draft Elliptic Curve Private Key Structure February 2009
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
1. Introduction
This document specifies a syntax and semantics for Elliptic Curve
(EC) private key information. EC private key information includes a
private key and parameters. Additionally, it may include the
corresponding public key. The syntax and semantics defined herein
are based on a similar syntax and semantics defined in Standards for
Efficient Cryptography Group (SECG) [SECG1].
Most Public Key Infrastructures (PKIs) mandate local key generation;
however, there are some PKIs that also support centralized key
generation (e.g., the public-private key pair is generated by a CA).
The structure defined in this document allows the entity that
generates the private and public keys to distribute the key pair and
the associated domain parameters.
A scenario in which this syntax is useful distributes EC private keys
using PrivateKeyInfo, as defined in PKCS #8 [RFC5208]. Distributing
an EC private key with PKCS#8 [RFC5208] involves including:
a) id-ecPublicKey, id-ecDH, or id-ecMQV (from [RFC5480]) with the
namedCurve as the parameters in the privateKeyAlgorithm field
b) ECPrivateKey in the PrivateKey field, which is an OCTET STRING.
When a public key is included, the publicKey field in ECPrivateKey is
used.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Elliptic Curve Private Key Format
This section gives the syntax for an EC private key. Computationally
an EC private key is an unsigned integer, but for representation, EC
private key information SHALL have ASN.1 type ECPrivateKey:
ECPrivateKey ::= SEQUENCE {
version INTEGER { ecPrivkeyVer1(1) } (ecPrivkeyVer1),
privateKey OCTET STRING,
parameters [0] ECParameters {{ NamedCurve }} OPTIONAL,
publicKey [1] BIT STRING OPTIONAL
}
Turner & Brown Expires August 3, 2010 [Page 2]

Internet-Draft Elliptic Curve Private Key Structure February 2009
The fields of type ECPrivateKey have the following meanings:
o version specifies the syntax version number of the elliptic curve
private key structure. For this version of the document, it SHALL
be set to ecPrivkeyVer1, which is of type INTEGER and whose value
is one (1).
o privateKey is the private key. It is an octet string of length
ceiling (log2(n/8)) (where n is the order of the curve) obtained
from the unsigned integer via the Integer-to-Octet-String-
Primitive (I2OSP) defined in [RFC3447].
o parameters specifies the elliptic curve domain parameters
associated to the private key. The type ECParameters are discussed
in [RFC5480]. As specified in [RFC5480], only the namedCurve
CHOICE is permitted. namedCurve is an object identifier that fully
identifies the required values for a particular set of elliptic
curve domain parameters. Though the ASN.1 indicates that the
parameters field is OPTIONAL, implementations that conform to this
document MUST always include the parameters field.
o publicKey contains the elliptic curve public key associated with
the private key in question. The format of the public key is
specified in Section 2.2 of [RFC5480]. Though the ASN.1 indicates
publicKey is OPTIONAL, implementations that conform to this
document SHOULD always include the publicKey field. The publicKey
field can be omitted when the public key has been distributed via
another mechanism, which is beyond the scope of this document.
Given the private key and the parameters the public key can always
be recomputed; this field exists as a convenience to the consumer.
4. Other Considerations
When generating a transfer encoding, generators SHOULD use DER
[X.690] and receivers SHOULD be prepared to handle BER [X.690] and
DER [X.690].
Section 1 described a format for transporting EC private keys (i.e.,
converting ECPrivateKey to PrivateKeyInfo [PKCS#8]); however, this
format can also be used for local storage.
Local storage of an unencrypted ECPrivateKey object is out of scope
of this document. However, one popular format uses the .pem file
extension. It is a PEM encoding, which is the Base64 encoding
[RFC4648], of the DER encoded ECPrivateKey object sandwiched between:
-----BEGIN EC PRIVATE KEY-----
-----END EC PRIVATE KEY-----
Turner & Brown Expires August 3, 2010 [Page 3]

Internet-Draft Elliptic Curve Private Key Structure February 2009
Another local storage format uses the .der file extension. In this
case, it is a DER [X.690] encoding of the ECPrivateKey object.
Local storage of an encrypted ECPrivateKey object is out of scope of
this document. However, ECPrivateKey should be the format for the
plaintext key being encrypted. DER [X.690] encoding ECPrivateKey
will promote interoperability if the key is encrypted for transport
to another party. PEM encoding the DER encoded ECPrivateKey is
common; "Proc-Type:" and "DEK-INFO:" fields [RFC1421] followed by the
DER Encoded ECPrivateKey are sandwiched between:
-----BEGIN EC PRIVATE KEY-----
-----END EC PRIVATE KEY-----
5. Security Considerations
This structure does not protect the EC private key information in any
way. This structure should be combined with a security protocol to
protect it.
Protection of the private-key information is vital to public-key
cryptography. The consequences of disclosure depends on the purpose
of the private key. If a private key is used for signature, then the
disclosure allows unauthorized signing. If a private key is used for
key management, then disclosure allows unauthorized parties to access
the managed keying material. The encryption algorithm used in the
encryption process must be as 'strong' as the key it is protecting.
6. IANA Considerations
None: All identifiers are already registered. Please remove this
section prior to publication as an RFC.
7. References
7.1. Normative References
[RFC1421] J. Linn, "Privacy Enhancement for Internet Electronic
Mail: Part I: Message Encryption and Authentication
Procedures," RFC 1421, February 1993.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3447] Kaliski, B., and J. Jonsson, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
Turner & Brown Expires August 3, 2010 [Page 4]